Lighting device

ABSTRACT

A lighting device ( 1 ) comprising at least one light source ( 3 ) as well as a light reflector disposed beside the light source ( 3 ) for reflection of at least part of the light radiated from the light source ( 3 ), a special feature being the fact that the light reflector comprises at least one light-transmitting element ( 4 ) bounding a space ( 5 ) at least in part, as well as a diffusely reflective powder ( 6 ) present inside said space ( 5 ).

The invention relates to a lighting device comprising at least one lightsource as well as a light reflector disposed beside the light source forreflection of at leastpart of the light radiated from the light source.It is noted that the term “beside” is understood to mean any spatialorientation, such as below, above, alongside, etc.

Such a lighting device is generally known. In a known lighting device,one or more light sources, such as TL tubes, are arranged in arectangular lighting housing provided with a reflecting rear wall andreflecting sidewalls. As a rule, a light-transmitting diffuser plate ispresent on the front side of the lighting housing for lateralhomogenisation of the light beam being emitted from the lighting device.Furthermore, one or more optically modulating foils may be present atthe side of the diffuser plate facing away from the light source, forexample for restricting the angular range of the light being emittedand/or imparting a particular polarization to the light being emitted.Such a lighting device is for example used for the back-lighting ofnon-emissive displays, such as LC displays, for the visual inspection ofX-ray photographs, for advertising billboards, for drawing tables, forflat luminaire tiles and/or for showcases for jewelry. The rear wall andthe side walls of the aforesaid lighting device are preferably diffuselyreflective, so that light that is not radiated from the light sourcedirectly towards the front side of the lighting device can subsequentlybe back-reflected via one or more diffuse reflections towards the frontof the lighting device, thus enabling light recycling. Normally, therear wall and the side walls of the lighting housing are made diffuselyreflective by coating them with a white coating layer consisting ofwhite particles in an organic binder matrix. The binder fixes theparticles in the coating and bonds the coating layer to the interiorwalls of the lighting housing. Usually, the coating has an opticallyrough surface facing towards the light source so as to prevent theoccurrence of specular reflections and to effect a proper homogenisationof the diffusely reflected light inside the lighting device. The precisepositioning of the coating layer relative to the position of the lightsource is of minor importance in this connection.

In another known lighting device, the light source, for example in theform of a CDM lamp, is present inside a lighting fixture which functionsas the housing of the light source and which partly surrounds the lightsource, whilst the lighting fixture itself comprises an essentiallyoptically smooth surface facing towards the light source. The shaping,orientation, and spacing of the said optically smooth surface around thelight source being carefully adjusted so as to obtain a controlledangular shaping of the emitted light beam from the lighting device. Saidsurface, which generally consists of aluminium, functions to specularlyreflect the light away from the CDM lamp within a particular angularrange, so as to realise a particular light intensity output of aspecific angular intensity distribution at a certain distance from theCDM lamp. One drawback of the aluminium surface of the lighting fixtureis that it absorbs a considerable amount of incident light thereon.Moreover, it is often difficult to realise a sufficiently smoothaluminium surface, and the optical smoothness of the aluminium surface,i.e. the ability to specularly reflect light, strongly decreases withthe passage of time, in particular owing to corrosion. As a result, thelight output of the known lighting device deteriorates in the course oftime. It is known, however, to overcome these drawbacks by providing thesurface of the lighting fixture that faces towards the light source witha white, smooth coating consisting of white, light-scattering particles,which are present in a binder matrix layer and which are bonded to thesurface of the lighting fixture therewith. Part of the light from theCDM lamp is specularly reflected by the coating, whilst another part isdiffusely reflected. Insofar as the coating provides specularreflection, it functions to restrict the angular intensity distributionrange of the light beam being emitted by the lighting device. Thespatial positioning of the smooth coating surrounding the light sourceis important in this connection; preferably it should faithfully followthe shape of the surface of the lighting fixture that faces towards thelight source.

One drawback of the aforesaid coating is that the application thereof isobjectionable for environmental reasons, since organic solvents aregenerally used, whilst on the other hand its application istime-consuming, because relatively thick coating layers are requiredwhich need to be dried carefully. Furthermore, the coating is usuallynot capable of withstanding the high operating temperatures of thelighting device, which may be in the order of a few hundred degreesCelsius, as a result of which discolouration and light absorption cantake place, which, with the passage of time, may lead to an undesirableshifting of the colour point and a reduced light output.

The object of the invention is to overcome the drawbacks of the priorart, and in order to accomplish that objective, a lighting device of thekind referred to in the introduction is characterized in that the lightreflector comprises at least one light-transmitting element bounding aspace at least in part and forming an inner side of the housing, as wellas a diffuse reflective powder present inside said space. The powder,which is in particular of the “free-flowing” type, preferably comprisescalcium halophosphate, calcium pyrophosphate, BaSO₄, MgO, YBO₃, TiO₂ orAl₂O₃ particles. Such a powder is resistant against high temperatures,whilst important chemical properties thereof do not deteriorate as aresult of being exposed to high temperatures, light and/or moisture. Thelight-transmitting element, if provided with optically smooth surfaces,on the one hand provides specular reflection of part of the incidentlight thereon, and on the other hand it directs another part of theincident light thereon towards the powder. The powder, in turn, providesdiffuse reflection (i.e. back-scattering) of said light back through thelight transmitting element into the direction of the light source. Thelight-transmitting element, if provided with optically roughenedsurfaces, counteracts the occurrence of specular reflections. Thepresence of the optically roughened, light-transmitting element and ofthe powder in that case ensure that a substantially complete diffusereflection is obtained.

In one preferred embodiment of a lighting device according to theinvention, the particles have an average diameter ranging between 0.1and 100 μm, in particular 5 to 20 μm. In order to obtain a“free-flowing” type powder, said particles are ideally mixed withfine-grained Al₂O₃ particles having an average diameter which rangesbetween 10 and 50 nm. The amount of the latter particles, also known asAlon-C (Degussa, Frankfurt), preferably ranges between 0.1 and 5 wt. %,in particular 0.5 to 3 wt. %.

In another preferred embodiment of a lighting device according to theinvention, the space has a thickness greater than or equal to 0.5 mm, inparticular greater than or equal to 1 mm, more in particular greaterthan or equal to 2 mm. Experiments have shown that these are the mostsuitable values, in the sense that, when a typical powder volume packingdensity of 30-60% in the space is present, a sufficient amount of powderis present to substantially completely reflect incident light thereon.

In another preferred embodiment of a lighting device according to theinvention, the light-transmitting element is a plate of glass or asynthetic material, which may or may not be flat. The space is inparticular bounded, at least in part, by the light-transmitting elementand by another light-transmitting element, so that a sandwich structureof two light-transmitting elements is obtained, the powder being presentin the space between the two light-transmitting elements. In anotherpreferred variant, the space is bounded, at least in part, by thelight-transmitting element and by a component of the lighting device, inparticular a housing of the light source or a lighting fixture of thelighting device.

In another preferred embodiment of a lighting device according to theinvention, the powder is mixed with colour pigments. This provides thedecorative effect whereby it appears as if (partially) coloured light isbeing emitted by the lamp.

In another preferred embodiment of a lighting device according to theinvention, the powder is incapable of absorbing light, at least lighthaving a wavelength in the visible wavelength range. Any loss of lightin this wavelength range due to absorption is thus prevented.

In another preferred embodiment of a lighting device according to theinvention, a surface of the light-transmitting element facing towardsthe light source is optically roughened. A surface of thelight-transmitting element facing towards the powder may likewise beoptically roughened. This enhances the diffuse nature of the light beingreflected by the diffuse light reflector.

The invention also relates to a method for manufacturing a lightingdevice, in which at least one light source and at least one lightingfixture are supplied and in which a light reflector is arranged besidethe light source for diffuse reflection of at least part of lightradiated from the light source and for specular reflection of at leastanother part of the light radiated from the light source so as toincrease light output of the lighting device and to restrict the angulardistribution of the intensity of the emitted light beam from thelighting device, characterized in that at least one light-transmittingelement bounding a space at least in part, as well as a diffuselyreflective powder present inside said space are used as the lightreflector, wherein the light-transmitting element comprises at least twosubstantially parallel, substantially optically smooth surfaces, andwherein the surface of the light-transmitting element that faces towardsthe light source extends substantially parallel to the surface of thelighting fixture facing the lamp.

The invention will now be explained in more detail with reference to twofigures illustrated in a drawing, which figures are a schematic sideelevation of two embodiments of a lighting device according to theinvention.

FIG. 1 shows a lighting device 1 comprising a lamp 3 fitted in alighting fixture 2. A diffuse light reflector comprising alight-transmitting glass plate 4 is arranged on the inner surface of thelighting fixture 2 in a continuous manner. The plate 4 is spatiallycurved such as to correspond to the spatial curvature of the innersurface of the lighting fixture 2, and it is arranged in a spaced-apartrelationship thereto so that an intermediate space 5 having a thicknessof 1 mm is formed between the glass plate 4 and the lighting fixture 2.The intermediate space 5 is filled with a powder 6 comprising calciumpyrophosphate. The calcium pyrophosphate particles are mixed with Al₂O₃particles so as to give the powder 6 its “free-flowing” character.Colour pigments may be added to the powder 6. The figure shows the pathof the rays of light emitted by the lamp 3, part of which issubsequently specularly reflected by the glass plate 4 (see, forexample, at location 7), whilst another part is diffusely reflected bythe powder 6 (see, for example, at location 8). Said diffuse reflectionmay be enhanced by optically roughening the surface of the glass plate 4that faces towards the lamp 3.

FIG. 2 refers to a light box comprising a housing 9 with four TL-tubes10 mounted therein, as well as a diffuser plate 11 placed on top of thehousing 9. The light reflector according to the invention comprises alight transmitting element 12 following the contours of the innersurface of the housing 9, such that an intermediate space 13 is formedfilled with the powder 14 in the way as mentioned in respect of FIG. 1.The light transmitting element 12 has an optically roughened surfacefacing towards TL-tubes 10, so that it prevents the occurrence ofspecular reflections and it provides diffuse reflections, for example atlocation 15. The powder 14 in turn provides diffuse reflection, forexample at locations 16 and 17.

The invention is not limited to the embodiment as discussed above, italso extends to other variants that fall within the scope of theappended claims. Thus, it will be apparent to a person skilled in theart that the glass plate 4 in FIG. 1 does not necessarily need tocontinuously follow the contours of the inner surface of the lightingfixture 2, but alternatively it may be arranged discretely(discontinuously) along said inner surface.

1. A lighting device comprising at least one light source as well as alight reflector disposed beside the light source for reflection of atleast part of the light radiated from the light source, characterized inthat the light reflector comprises at least one light-transmittingelement bounding a space at least in part, as well as a diffuselyreflective powder present inside said space.
 2. A lighting deviceaccording to claim 1, wherein said powder comprises calciumhalophosphate, calcium pyrophosphate, BaSO₄, MgO, YBO₃, TiO₂ or Al₂O₃particles.
 3. A lighting device according to claim 2, wherein theparticles have an average diameter ranging between 0.1 and 100 μm, inparticular 5 to 20 μm.
 4. A lighting device according to claim 2,wherein said particles are mixed with fine-grained Al₂O₃ particleshaving an average diameter which ranges between 10 and 50 nm.
 5. Alighting device according to claim 4, wherein the amount of fine-grainedAl₂O₃ particles having an average diameter ranging between 10 and 50 nmranges between 0.1 and 5 wt. %, in particular 0.5 to 3 wt. %.
 6. Alighting device according to claim 1, wherein said space has a thicknessgreater than or equal to 0.5 mm, in particular greater than or equal to1 mm, more in particular greater than or equal to 2 mm.
 7. A lightingdevice according to claim 1, wherein the light-transmitting element is aplate of glass or a synthetic material.
 8. A lighting device accordingto claim 1, wherein said space is bounded, at least in part, by saidlight-transmitting element and by another light-transmitting element. 9.A lighting device according to claim 1, wherein the space is bounded, atleast in part, by the light-transmitting element and by a component ofthe lighting device, in particular a housing of the light source or alighting fixture of the lighting device.
 10. A lighting device accordingto claim 1, wherein said powder is mixed with colour pigments.
 11. Alighting device according to claim 1, wherein said powder is a“free-flowing” type powder.
 12. A lighting device according to claim 1,wherein the powder is incapable of absorbing light, at least lighthaving a wavelength in the visible wavelength range.
 13. A lightingdevice according to claim 1, wherein a surface of the light-transmittingelement facing towards the light source is optically roughened.
 14. Alighting device according claim 13, wherein a surface of thelight-transmitting element facing towards the powder is likewiseoptically roughened.
 15. A method for manufacturing a lighting device,in which at least one light source and at least one lighting fixture aresupplied and in which a light reflector is arranged beside the lightsource for diffuse reflection of at least part of light radiated fromthe light source and for specular reflection of at least another part ofthe light radiated from the light source so as to increase light outputof the lighting device and to restrict the angular distribution of theintensity of the emitted light beam from the lighting device,characterized in that at least one light-transmitting element bounding aspace at least in part, as well as a diffusely reflective powder presentinside said space are used as the light reflector, wherein thelight-transmitting element comprises at least two substantiallyparallel, substantially optically smooth surfaces, and wherein thesurface of the light-transmitting element that faces towards the lightsource extends substantially parallel to the surface of the lightingfixture facing the lamp.